The standard master antenna dual-input RF system is typically designed for use by multiple radio stations operating both digital and analog transmitters. The antenna is typically provided with two wide-band ports, one designated analog and the other digital for combining in the antenna. Each station is provided its own analog or analog and digital IBOC (in-band-on-channel) combiner module to restrict transmitter-to-transmitter interference.
In single antenna input configurations, the multiplexing of combiner modules leaves a port available for future station expansion. The broad band port is always terminated with a load resister and is assumed to be available for emergency use. High levels of RF voltage may be present at this port because of summed energy from other transmitter combiner modules and reflected power from a mismatched antenna. It should always be assumed that there will be energy present at the broad band port and that port access must be restricted.
Intermodulation products are unintended frequencies generated within an RF system that potentially cause interference to other operations. To prevent intermodulation products and protect against overloading a directly connected station transmitter, a band pass filter is generally required and used to isolate the transmitter. The filter input port is only suitable for a single station.
Isolating the port for ease of access and flexible frequency use is an expensive problem. Stray voltages present at the multiplexer broad band port can be diminished by installing a circulator.
A circulator is a unidirectional 3 port device which passes forward energy undiminished and unshifted in phase from its input to its output port while directing reflected and incoming signals at its output port to a connected resistive load at its third port. Circulators are very narrow band devices and do not have wide band frequency responses required for master antenna systems. This is important because a circulator has decreased performance at frequencies other than the operating frequency within the FM operating band. Spurious energy reflected back to the output of the circulator must be absorbed by the circulator load resister which can cause considerable heating. Circulators can easily become saturated by the high forward power from an analog or digital IBOC RF injected source and, along with the summed energy of reflected waves from the master antenna station participants, become unstable. The transmitter isolation gained from the use of the circulator must be maintained under all types of operating conditions otherwise intermodulation products may be created potentially causing additional problems.
When digital IBOC was introduced, a level of −20 dBc (0.01 of the analog signal) was expected to provide similar coverage to the existing analog operations; however, it has been necessary to increase the digital IBOC power levels up to −10 dBc (0.10 of the analog signal) to achieve satisfactory coverage. Digital transmitters have always been susceptible to poor isolation from analog transmitters. Poor isolation causes intermodulation products to be generated, exceeding FCC allowable emissions. In order to increase isolation between analog and digital transmitters, it is common to use a circulator on the output of the digital transmitter. The circulator provides two benefits to the broadcaster. The first is increased isolation from the analog transmitter, thus reducing the reflected power seen by the digital transmitter and extending its life. The second benefit is that the circulator provides an extra failsafe for the digital transmitter in the event of antenna failure. Failure can occur from a burn out or a lightning strike. During upsets, it is possible for excessive analog power to be coupled to a digital transmission line run, resulting in digital transmitter failure while the analog transmitter is unaffected. The circulator helps guard against this possibility.
Current digital power levels are well within the limits of the existing circulators; however, as −10 dBc applications are being implemented, many of the circulators are not rated for the power increase. When it is overpowered, the circulator may lose its ability to isolate ports and allow RF power a path to the digital transmitter instead of dissipating the power in its resistive reject load. The circulator may also fail if the resistive load is over powered and burns, thus allowing all reflected power to be directed back to the transmitter with only a small loss. In both cases, the analog transmitter is susceptible to digital power coupling into the analog transmission line, causing the analog transmitter to see excessive reflected power.
FIG. 1 demonstrates the performance of a single typical prior art circulator. The circulator is inherently broad band if a VSWR of 1.5:1 is specified as broad band; however, when that specification is tightened to a VSWR of 1.1:1 the circulator appears to be narrow banded. For purposes of illustration, this circulator is tuned for 101 MHz. At 101 MHz, VSWR and isolation traces reach a compromise for best performance.
Circulators are sensitive to environmental changes. Air conditioning in a room can affect performance significantly. Frequently, when first put into operation, it is necessary to “hot-tune” the circulator as it warms and drifts from its tuned frequency. This method usually takes several hours until the circulator's temperature stabilizes.